The words "die" and "chip", and the words "detector" and "sensor", are used interchangeably herein.
Opto-isolators are well known in the electronics art. Referring to FIG. 1, typical semiconductor opto-isolator 1 comprises light emitting chip 2 mounted on electrical input leads 3 and, spaced apart and electrically isolated therefrom, light sensing or detecting chip 4 mounted on output leads 5. The space between the chips is typically filled with an optically transparent organic dielectric 6, e.g., a die coat, which serves as a "light pipe" to direct light 7 from light emitting chip 2 toward light detecting chip 4.
Die coat 6 is surrounded by opaque outer covering 8, typically a molded plastic encapsulation containing various fillers. No electrical connection exists between input chip 2 and output chip 4. Such opto-isolators provide a very high degree of electrical isolation between input leads 3 and output leads 5. Light emitting diodes (LED's) are examples of typical light emitter chips. Detector chips typically comprise one or more semiconductor elements, as for example, diodes, resistors, transistors, thyristors, TRIACS and/or combinations thereof adapted for optical input. U.S. Pat. Nos. 4,458,408 and 4,396,932 describe a typical prior art light activated detector device suitable for use in an opto-isolator and are incorporated herein by reference.
A particular problem arises when opto-isolator 1 is intended to stand off voltage 9, as for example a voltage.gtoreq.100 volts and especially a voltage.gtoreq.1000 volts. In this situation, while input optical emitter 2 and output optical detector 4 may each be operating at comparatively low voltages, e.g., 3-15 volts, there is a substantially larger stand-off voltage 9 between input leads 3 and output leads 5, and therefore between emitter die 2 and detector die 4.
Stand-off voltage 9 produces an electric field between input emitter die 2 and output sensor die 4 within opto-isolator 1. Mobile ions 10 may exist within die coat 6 or may enter die coat 6 from the surrounding encapsulation 8 despite the best efforts to avoid them. If the electric field in die coat 6 is large enough and/or the temperature high enough, mobile ions 10 will move and may pile up against the surface of emitter 2 and/or detector 4 or both. In general the mobility of such ions increases with increasing temperature.
To a first approximation, the input-output stand-off voltage creates an electric field at the die surfaces which is approximately perpendicular thereto. This is to be distinguished from the electric field which result from voltage applied to the devices themselves. For example, sometimes detector chip 4 must also support a large blocking voltage within or across the chip. This is typically the case where detector chip 4 is a high voltage thyristor or TRIAC. To a first approximation, the internal blocking voltage within the detector creates at the detector surface an electric field which is approximately parallel to the die surface. Thus, the input-output stand-off voltage and the voltages applied separately to the individual emitter or detector die create approximately orthogonal electric fields and have different effects.
Most light emitting chips are comparatively insensitive to surface ions or other surface charge and the amount of light emitted for a given electrical input is not substantially affected by an accumulation of charge or foreign ions on the emitter surface.
With optical detectors the situation is different. Many types of desirable optical detectors, as for example, photo-diodes, resistors, transistors, thyristors, TRIACS and the like, can be much affected by surface charge. Their electrical properties, e.g., impedance, transconductance, on-off state and the like, can change dramatically depending upon whether or not charged particles are present on certain regions of their surfaces. If ions of a particular polarity pile up on the surface of such detector chips in response to the approximately perpendicular, stand-off electric field, the transfer characteristic of the opto-isolator device as a whole, e.g., the electrical output impedance of the optical detector versus the electrical input signal to the optical emitter, may be unstable, i.e., change with time of operation.
The above-described instability is a significant problem in opto-isolators exposed to stand-off voltages of about 10.sup.3 volts or more, particularly if they must operate at elevated temperatures. It has been found that instability due to mobile ions in the die coat is very pronounced in opto-isolators in which the detector is made positive with respect to the emitter, that is, when the direction of the approximately perpendicular, stand-off electric field at the surface is such as to attract negative ions to the detector. This die coat mobile ion instability is significant even with detectors which are surface passivated with stable dielectrics, such as silicon oxide, silicon nitride and combinations thereof.
Various techniques have been used in the prior art to overcome this stand-off voltage instability problem. For example, the metallization used to contact the various regions of the detector is extended out onto the surface passivation layer over the locations where the perimeter of the detector PN junctions intersects the die surface. One or more metal layers may be used for this purpose, connected to one or another of the leads of the detector. This arrangement terminates the perpendicular electric field lines on the metal rather than on the semiconductor and fixes the electric potential above the surface junction perimeter at the metal potential independent of whether ions from the die coat accumulating on the surface. This makes the detector less sensitive to stand-off voltage induced ion migration effects. The large area metallization also precludes any lateral surface electric fields over the metal covered regions which might prompt lateral migration of die coat mobile ions across the die surface.
A disadvantage of having the metallization overlap virtually the entire PN junction perimeter is that less light reaches the light sensitive regions of the detector. This reduces the opto-isolator sensitivity. In order to achieve the same detector output impedance, the input LED must be driven harder.
For example, consider an opto-isolator having an output detector without significant overlapping metal that has an electrical output impedance of about 10.sup.6 ohms in the "off-state", e.g, no LED drive, and an output impedance of about 10.sup.0 ohms in the "on-state" at, say, 10 milliamps LED drive. If, in order to obtain high stand-off voltage stability, the metallization area of the detector is increased to cover more of the PN junction perimeter and cuts the optically active area of the detector by half, then the input LED drive must be increased to about 20 milliamperes to obtain the same 10.sup.0 ohms "on-state" impedance.
Unfortunately, the operating life of LEDs decreases with increasing drive. Thus, use of larger area overlapping metal to avoid stand-off voltage instability arising from die coat ion migration, not only degrades the overall opto-isolator sensitivity and transfer characteristic, but also leads to shorter emitter life for a given detector output impedance. In general, semiconductor opto-isolators are not repairable. Hence, degradation or failure of the emitter usually means that the opto-isolator must be discarded.
A further disadvantage of the overlapping metal arrangement is that it is only practical with simple devices or circuits. As the detector complexity increases, i.e., beyond about twenty semiconductor devices, it is more and more difficult to provide the proper junction perimeter overlap with a single metal layer. Multiple layers of metal are sometimes used but these increase the fabrication complexity and cost. For some complex integrated detector circuits, even multiple layers of metal may be impractical. Accordingly, there is an ongoing need for improved opto-isolators which avoid these and other problems.